. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] THE MAGNETIC FIELD OVER THE OCEANS 90° 75° 60° 45° 30° 187 0,45 Fig. 7. Section from the Hydrographic Office chart of the total field in 1955 (No. 1703). Wave-length (miles) 100 5040 30 20 15 10 1500 1 1 III i o c o E a ^ J \ JC \ 7 1 Q. 1000 - ^ J /\ 1 0 M E x^ 13 /o 1 1 X^ \ 1 "*^ S- 500 — \ I ^^--^ \_ 5 \ \ /f'^v "*** ""^ o \ \i n C]^ Q ***— ^ a. 1 a ; l,.»,,,<^~ Wave number (1/miles) Fig. 8. Power spectrum of the magnetic field on a prof
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] THE MAGNETIC FIELD OVER THE OCEANS 90° 75° 60° 45° 30° 187 0,45 Fig. 7. Section from the Hydrographic Office chart of the total field in 1955 (No. 1703). Wave-length (miles) 100 5040 30 20 15 10 1500 1 1 III i o c o E a ^ J \ JC \ 7 1 Q. 1000 - ^ J /\ 1 0 M E x^ 13 /o 1 1 X^ \ 1 "*^ S- 500 — \ I ^^--^ \_ 5 \ \ /f'^v "*** ""^ o \ \i n C]^ Q ***— ^ a. 1 a ; l,.»,,,<^~ Wave number (1/miles) Fig. 8. Power spectrum of the magnetic field on a profile in the north-western Atlantic. The dashed line represents the spectrum which would be observed if the spectrum at the sea bottom were "white". Since measurements at sea are taken at or above the surface of the sea and many thousands of metres above the ocean bottom, local anomalies will be smoothed and attenuated. If the anomalies are represented by a space spectrum, a component of wave-length A will be reduced in the ratio exp ( —2TrhlX) if it is measured at a height h above the bottom. In water of 5000-m depth the attenuation is by a factor of over 2 for all wave-lengths below 14 km. There is, therefore, a very substantial loss of detail as compared to a survey conducted on land. A smoothed power spectrum taken by a ship along an east-west profile in a depth of about 2500 fm in the eastern basin of the North Atlantic is shown in Fig. 8. The profile is 379 miles in length and runs along latitude. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hill, M. N. (Maurice Neville), 1919-. New York : Interscience Pub.
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectoceanbottom
